Vinylidene fluoride (VDF) copolymers comprising recurring units derived from trifluoroethylene (TrFE) monomer have been used extensively in the electronics packaging market due to their ease of processing, chemical inertness and attractive ferroelectric, piezoelectric, pyroelectric and dielectric properties.
As is well known, the term piezoelectric means the ability of a material to exchange electrical for mechanical energy and vice versa and the electromechanical response is believed to be essentially associated with dimensional changes during deformation or pressure oscillation. The piezoelectric effect is reversible in that materials exhibiting the direct piezoelectric effect (the production of electricity when stress is applied) also exhibit the converse piezoelectric effect (the production of stress and/or strain when an electric field is applied).
Ferroelectricity is the property of a material whereby this latter exhibits a spontaneous electric polarization, the direction of which can be switched between equivalent states by the application of an external electric field.
Pyroelectricity is the ability of certain materials to generate an electrical potential upon heating or cooling. Actually, as a result of this change in temperature, positive and negative charges move to opposite ends through migration (i.e. the material becomes polarized) and hence an electrical potential is established.
It is generally understood that piezo-, pyro-, ferro-electricity in copolymers of VDF with TrFE is related to a particular crystalline habit, so called beta-phase, wherein hydrogen and fluorine atoms are arranged to give maximum dipole moment per unit cell.
Copolymers comprising recurring units derived from vinylidene fluoride and trifluoroethylene are typically provided as semicrystalline copolymers which can be shaped or formed into semicrystalline, essentially unoriented and unstretched, thermoplastic film or sheet or tubular-constructed product via well known processing methods such as extrusion, injection moulding, compression moulding and solvent casting.
Nevertheless, more recently, developments of thin film electronic devices and/or assemblies of ferroelectric polymer layers in three-dimensional arrays for increasing e.g. memory density have called for different processing techniques, requiring notably ability of the polymer to be patterned according to lithographic techniques and/or for layers there from to be stacked with annealing treatment on newly formed layer not affecting previously deposited layers.
Within this scenario, thus, cross-linking (elsewhere referred to as ‘curing’), which is one of the most known techniques in polymer science to stabilize shape and fix structures, has been the technique of choice for accessing these needs.
Solutions have thus been proposed for conferring to VDF-TrFE copolymers cross-linking or curing ability. Among those solutions, use of azide-containing coupling agents, because of their ability of inserting into carbon-hydrogen bonds under thermal or UV treatment, and yet of their relative robustness, has been considered. So, VAN BREEMEN, A. J. J. M., et al. “Photocrosslinking of ferroelectric polymers and its application in three-dimensional memory arrays”. Appl. Phys. Lett. 2011, vol. 98, p. 183302. and US 2007/166838 (PHILIPS ELECTRONICS NORTH AMERICA CORPORATION) discloses a photolithography process designed to provide access to three-dimensional memory arrays, said process involving the photocrosslinking of VdF-TrFE polymers using as cross-linking agent 2,6-bis(4-azidebenzylidene)-4-methylcyclohexanone.
Similarly, WO 2005/064705 (KONINKLIJKE PHILIPS ELECTRONICS N.V.) Jul. 14, 2005 discloses patterning by means of photolithography of fluorinated ferroelectric polymer layers, such as those derived from VdF-TrFE (random) copolymers, by addition of a photosensitive cross-linker, such as, e.g., a bis-azide, to a fluorinated polymer spin-coat solution. No mention is made therein of suitable specific bis-azide derivatives.
Nevertheless, this procedure requires a quite delicate metering and mixing of the added azide-containing curing agent into the VDF-TrFE polymer matrix for achieving a reasonably homogeneous mixture; incompatibility between these components might lead to uneven distribution of crosslinking density in cured compound, with regions being left nearly unmodified and regions possessing high crosslinking modification so that piezo-, pyro-, ferro-electricity properties might be affected.
There is thus still a need in the art for VDF/TrFE copolymer materials which can efficiently undergo crosslinking under thermal or UV exposure conditions, yielding a uniformly cured material which still maintains outstanding piezoelectric, ferroelectric, pyroelectric and dielectric properties.
On the other side, the incorporation of azide-containing monomers in fluoropolymer chain has been described in the art, in particular for fluoroelastomers.
Thus, U.S. Pat. No. 6,365,693 (DUPONT DOW ELASTOMERS LLC) Apr. 2, 2002 discloses the incorporation of compounds of formula:CX1X2═CX—(O)p—Rf—(CH2)n—S(O)qN3 wherein: X, X1 and X2 are independently H or F, p is 0 or 1, n is 0-4, q is 1 or 2, Rf is a perfluoroalkyl or a perfluoroalkoxy group, as cure-site monomers in copolymers of fluorinated monomers, at least one of them being selected from VDF, TFE and chlorotrifluoroethylene (CTFE).
Similarly, US 2010/032422 (DUPONT PERFORMANCE ELASTOMERS L.L.C.) discloses a fluoroelastomer comprising copolymerized units of:                a first monomer selected from vinylidene fluoride and tetrafluoroethylene, and        a cure site monomer having a cure site selected from azide, sulfonyl azide and carbonyl azide groups. Non-limitative examples of suitable sulfonyl azide cure site monomers include the followings: CF2═CFOCF2CF(CF3)OCF2CF2—SO2N3, CF2═CFOCF2CF2—SO2N3, CF2═CFOCF2CF2CF2—SO2N3 and CF2═CFOCF2CF2CF2CF2—SO2N3.        